Fiber-optic communication networks are experiencing rapidly increasing growth of capacity. This capacity growth is satisfied both through increased data rate of individual wavelength division multiplexed (WDM) channels (e.g., 10 Gbps to 40 Gbps to 100 Gbps and beyond) and with decreasing spacing between WDM channels, i.e. decreased spectral spacing such as 100 GHz to 50 GHz. Both of these approaches lead to an increased spectral overlap between channels thereby increasing linear crosstalk between channels. This problem has been recognized, as it leads to impairments when multiple closely-spaced WDM channels are added at Terminals or optical add-drop multiplexed (OADM) nodes, i.e. optical-to-electrical-to-optical (OEO) points in optical networks. Previous attempts to deal with this problem can be grouped into two categories: narrow-band optical filters and digital signal processing.
The first category uses narrow-band optical filters at transmitters to suppress spectral overlap between adjacent optical WDM channels. For example, P. J. Winzer, et al, ‘Coherent crosstalk in ultra-dense WDM systems,” J. Lightwave Techn., vol. 23, no. 4, April 2005, discuss linear cross talk penalties, and shows how these can lead to bursty error behavior. G. Bosco et al., “On the use of NRZ, RZ, and CSRZ modulation at 40 Gb/s with narrow DWDM channel spacing,” J. Lightw. Technol., vol. 20, no. 9, pp. 1694-1704, September 2002, consider optical transmitter filter optimization, such that cross talk is minimized, while associated transmitted signal distortions are minimized. Similarly, Bosco et al. and A. Hodzic et al., “Optimized filtering for 40-Gb/s/ch-based DWDM transmission systems over standard single-mode fiber,” IEEE Photon. Technol. Lett., vol. 15, no. 7, pp. 1002-1004, July 2003, attempt to optimize bandwidth of transmitter and receiver-side optical filters for best overall WDM channel performance.
Spectral control of WDM channels by tight optical filtering has several substantial drawbacks. Optical filters are fundamentally difficult to fabricate with precise and repeatable control of pass band amplitude and phase response. The filters are required to operate at an optical center frequency in the vicinity of 193 THz, but require bandwidths of approximately 40 GHz leading to approximately a 5000:1 aspect ratio. Also, optical filters center frequency tends to drift due to aging and temperature effects, which leads to variable impact on WDM channel performance. WDM channel laser frequency can drift which again misaligns the channel relative to the optical filter and leads to a variable impact on WDM channel performance. Additionally, the requirement for reconfigurable networks implies that a WDM channel laser frequency can change under software control. Correspondingly, optical filters have to dynamically track changes in the laser frequency, while maintaining all the other performance characteristics. Finally, optical filters, especially tunable ones, are generally physically large and expensive devices.
The second category uses sophisticated digital signal processing to partition the incoming data stream into many parallel streams, each digitally modulated onto a separate closely spaced carrier. Such approaches are known as Orthogonal Frequency Division Multiplexing (OFDM), and can produce compact signal spectra with very sharp roll-off and minimal linear cross talk Spectral control via digital signal processing, such as generation of OFDM signals also has associated drawbacks. It requires sophisticated digital circuit implementations, as well as very high speed Digital-to-Analog Converters (DAC). Such circuits require millions of gates and have correspondingly high complexity, cost and power consumption, even in latest generation Complementary metal-oxide-semiconductor (CMOS) technology. Also, OFDM modulation generally requires additional overhead for cyclic prefix, pilot tones for phase synchronization, and training sequences. All of these effectively expand signal bandwidth, increasing associated interference.
As optical networks move to data rates in excess of 100 Gbps with spectral spacing of 100 GHz or less, single-carrier implementations in WDM optical networks have a need for systems and methods to suppress linear crosstalk while overcoming the aforementioned limitations described above.